Solid, absolutely bubble-free shaped articles, for example, (those made of reaction-cast polyurethane elastomers) are produced extensively in open molds, for example, by so-called open manual casting. In this process, the flowable, slightly activated multi-component reaction mixture is poured slowly into the mold without whirl or turbulence so as to avoid the trapping of bubbles. Any air bubbles possibly occurring on the free surface are destroyed by burning them off with a burner flame.
Attempts have been made to produce solid shaped articles which are free from bubbles and blow holes in closed molds, in particular, from highly reactive multi-component reaction mixtures to allow short cycle times for the economic mass production of such shaped articles which are usually of large volume. The use of self-cleaning, forced control fitted injection mix heads and the knowledge about the laminar filling operation avoiding the trapping of air by building up a closed flow front through the reaction mixture entering the mold cavity at high speed, also allow for the production of shaped articles which are usually thin-walled from highly reactive multi-component reaction mixtures of this type (for example, based on polyurethane).
However, the metering instruments with injection mixers used in the past allowed only a predetermined total quantity of mixture to be metered. It is known that molds for the production of identically shaped articles of the type used regularly for mass production in, for example, the automotive industry, have tolerances in their mold cavities due to production. This means that the volumes of the cavities of identical molds may differ slightly from each other. In order to eliminate any differences in volume during the successive filling of these molds using the same metering unit, the molds are charged with too much reaction mixture and the excess quantity is carried off or collected by means of a separating plane, ventilation ducts or in lost riser gates. This process has the disadvantage of material waste and of the subsequent work needed to remove the expelled material. The problem arises, in particular, during the production of relatively large shaped articles for which purpose a large quantity of reaction mixture must be introduced into the mold within a few seconds due to the short setting times. Although the air contained in the mold cavity can escape freely at the beginning, the area available for ventilation generally decreases to an even greater extent as the degree of filling increases. In this process, the ventilation problem is aggravated, particularly if the mold cavity is designed in such a way that the cross-section of the portion to be filled last contracts and the flow front thus progresses more quickly. The danger of trapping air then arises.
Finally, the inaccuracy in switching by means of the time clocks used for controlling the metering process has an adverse effect due to the short metering times. The time clocks available at present have, for example, an error of a maximum of 0.5%, related to the final value set. If this final value is, for example, 3 seconds, an error of 1.5% is already produced after a metering time of 1 second. The total error is increased by additional timing errors which arise, for example, due to the conversion of the electrical control pulse by means of the reversal of a hydraulic valve and the mechanical reversal of the control members, such as slides or valves, thus triggered to actuate the mixer head. Variations in the temperature of the hydraulic fluid can also increase the error. A total error of 2% and above related to the mold filling time between reversal of the mixer from circulation into the mixing position and from the mixing position back into the circulation position after the filling operation can very easily occur in this way. During the production of shaped articles having a quite conventional volume of, for example 5 lit., an overall error in the filling volume of about 100 ml is produced over a mold filling time of 1 second.
The above-mentioned over-filling of a mold is also coupled with pressure rise in the mold cavity and thus also in the injection mixing chamber communicating with the mold cavity. This pressure rise inevitably influences the mixing conditions and can impair the mixed products.
The object of the invention is to ensure exact filling of cavities, in particular, of molds, without bubbles and blow holes, with simplified ventilation of the cavity toward the end of the mold filling operation, wherein a pressure which can optionally be preselected can preferably be monitored and controlled with relation to time in the entire system extending from the mixing chamber to the cavity.